Mercury discharge lamp comprising magnesium halide

ABSTRACT

A mercury vapour discharge lamp primarily for producing ultraviolet radiation has an atmosphere which in addition to mercury vapour and an inert gas filling for starting an electron discharge, includes magnesium in the form of a halide. The mercury concentraiton is below 0.5 mg/cc of lamp volume, and the magnesium to mercury weight ratio is between 0.002:1 and 0.1:1. The magnesium is introduced into the lamp as magnesium metal together with another metal halide, preferably a mercuric halide.

United StatesPatent n91 Furmidge et al.

[ Feb. 18, 1975 [22] Filed:

[ MERCURY DISCHARGE LAMP COMPRISING MAGNESIUM HALIDE [75] Inventors: Kenneth Frederick Furmidge; Eric John George Beeson, both of London, England [73] Assignee: Thorn Electrical Industries Limited,

London, England Nov. 21, 1973 [21] Appl. No.: 418,122

Related U.S. Application Data [63] Continuation-impart of Ser, No. 256,006, May 23,

1972, abandoned.

[30] Foreign Application Priority Data July 5, 1973 [52] U.S. Cl 313/485, 313/225, 313/229 [51] Int. Cl.. I-I0 lj l/54, HOlj 61/20 [58] Field of Search 313/109, 184, 225, 228,

[56] References Cited UNITED STATES PATENTS 3,319,119 5/1967 Rendina 313/ 225 X Great Britain 32104/73' 3,351,798 11/1967 Bauer 313/229 X 3,398,312 8/1968 Edris et al.. 313/225 3,431,447 3/1969 Larson 313/229 X 3,480,819 11/1969 Faria et al i. 313/109 3,521,111 7/1970 Tsuchihashi et a. 313/225 X OTHER PUBLICATIONS Reiling, Characteristics of Mercury Vapor-Metallic Iodide Arc Lamps, Journal of the Optical Society of America, Vol. 54, No. 4, Apr. 64.

Primary Examiner-Paul L. Gensler Attorney, Agent, or Firm-Dike, Bronstein, Roberts, Cushman & Pfund [57] ABSTRACT A mercury vapour discharge lamp primarily for producing ultra-violet radiation has an atmosphere which in addition to mercury vapour and an inert gas filling 30 Claims, 2 Drawing Figures MERCURY DISCHARGE LAMP COMPRISING MAGNESIUM HALIDE CROSS-REFERENCE TO' RELATED APPLICATION This application is a co'ntinuation-in-part of our application Ser. No. 256,006', filed May 23', I972, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to mercury vapour discharge lamps for producing ultra-violet radiation.

Such ultra-violet-emitting lamps have particular util ity in providing an 'energy'source for curing photosensitive printing inks. For such purposes there is a requirement for a reasonably broadband high power ultraviolet source which is efficient in its conversion of electrical energy to ultra-violet radiation.

In the ultra-violet waveband ranges 200 nm to 300 nm and 360 nm to 390 nm it is however difficult to obtain a broad band of radiation from discharge lamps. The most widely used lamps for providing radiant energy in the short and long ultra-violet bands covering 200 nm to 390 nm depend on the excitation of mercury alone. The proportion of energy radiated in this waveband is greatly dependent upon the mercury vapour pressure and in the waveband 200 nm to 300 nm the resonant line at 254 hm is the most effective, and in the 360 nm to 390 nm waveband the 366 nm line is the most effective. However, because of the low operating pressure and low envelope temperature necessary for generating the shorter wavelength ultra-violet radiation it has not been possible to produce a high efficiency, high power lamp of small volume to provide a high concentration of energy in the desired wavebands.

It has previously been proposed to include additives in mercury dischargelamp envelopes to increase the lamp efficiency inthe visiblewaveband, but in such prior proposals there has been a corresponding reduction of emission in the ultra-violet waveband.

More particularly, it has been proposed by G.H. Reiling in an article Characteristics of Mercury Vapour- Metallic Iodide Arc Lampspublished in the Journal of the-Optical Society of America, Vol. 54, No. 4, April I964, pages 532 to 540, to add magnesium to a mercury-iodide lamp ofmediurn pressure, i.e.-, with l to lo atmospheres of mercury, to improve the colour rendition of the lamp in the visible spectrum. In this respect Reiling found magnesium was unattractive for his purposes as the efficiency of the lamp was poor. Reiling also discusses in his US. Pat. No. 3,234,421 the addition of alkaline earth metals in general and notes that these lead to a reduction in ultra-violet radiation.

US Pat. No. 3,319,119 to Rendina mentions the use of a large group of metal halides of which magnesium is one in a lamp which is required to provide emission in the visible waveband with sharp' spectral lines which are not subject to line broadening. However-such-a lamp would not provide a high efficiency broadband ultra-violet spectrum.

Accordingly it is an object of the invention to provide a mercury halide discharge lamp with enhanced broadband emission in the ultra-violet region, which operates with high conversion efficiency, which is small in size, and which does not require unduly high voltages.

SUMMARY OF THE INVENTION We have found that by adding magnesium in the form of a halide to a-lampwe can extend and increase the ultra-violet radiation either side of the more important mercury radiation at 254 nm and 366 nm. The lamp thus comprises mercury, magnesium in the form of a halide such as the iodide, and an inert gas filling for starting an electron discharge. The magnesium iodide is conveniently introduced into the lamp as magnesium metal together with mercuric iodide. The total amount of mercury in the lamp is below 0.5 mg per cc of internal la'mp volume which provides a lower pressure than is customary. The weight of magnesium is preferably about 2 to 5% of the weight of mercury but can range between 0.2% and 10%.

The relatively low mercury concentration leads to high current values through the lamp and it is found that this leads to utilisation effectively of the whole cross-section of the lamp to carry the current leading in this instance to greater lamp efficiency.

When magnesium is excited within the mercury plasma additional bands of radiation occur between 277 nm and 285' nm and also at 383 nm. Thus the added magnesium contributes to additional lines with those of mercury. The lamp can be used to activate photoresponsive and photochemical processes, having peak responses in the ultra-violet waveband, to achieve a substantial reduction in exposure time.

In addition we have found that the phosphor coatings such as used on the outer jackets of conventional high pressure mercury vapour lamps will also be excited by the radiation from the magnesium excitation to give an enhanced output from the phosphor. Further, there is also additional line radiation occurring outside the ultra-violet band emitting visible light in the blue-green region at 516 nm to 528 nm, which will contribute to an improvement in the colour rendition property of a lamp and its colour appearance. Thus the ultra-violet emitting lamp can be used to provide visible radiation.

It is possible to add trace additives of certain other metal halides, preferably gallium halide, to contribute to radiation in the ultra-violet wavebands and, if the lamp has a phosphor-coated outer jacket, to improve the colour quality of the visible spectrum produced thereby. The additives are chosen so as not to significantly diminish the magnesium radiation or result in the absorption of the ultra-violet radiation.

BRIEF DESCRIPTION OF THE DRAWINGS Further objects and advantages of the invention will be seen from the following exemplary description of the invention taken with reference to the accompanying drawings, in which:

FIG. 1 is a general view of a discharge lamp; and

FIG. 2 is a graph illustrating the ultra-violet emission spectrum of a lamp.

DESCRIPTION OF THE PREFERRED EMBODIMENTS ii. a proportionate amount of magnesium, in the form of a halide, which is excited to provide increased emission in the waveband 200 nm to 390 nm, and

iiixan inert gas filling for starting an electron discharge.

The magnesium halide is conveniently introduced into the lamp in the form of magnesium and another metal halide, preferably a mercuric halide. Examples of such a lamp will now be described.

EXAMPLE I One such lamp rated at 1,200 watts has a fused silica envelope with an internal bore of 14 mm and an arc gap length of 190 mm. The electrodes are. of tungsten and contain a percentage of thorium or added oxide, or have a fused thorium metal tip, to provide a source of electron emission. The electrodes are connected to thin molybdenum foils which pass through the hermetic seal and terminate in lead wires each with a small cap assembly to which is made the electrical connections.

The lamp contains mg of mercury, 0.5 mg of magnesium and about 10 mg of mercuric iodide to provide a pressure where the total ultra-violet energy emission is high relative to a lamp designed to operate over the visible part of the spectrum. In order to establish an arc discharge a gas filling of argon is added with a filling pressure of to 50 torr. The quantities can be adjusted over a range, for example the partial pressure of mercury is of the order of 1 atmosphere but can be reduced to about 5 1. atmospheres, with proportional adjustments to the other additives. The ratio by weight of magnesium, in the form of a halide, to mercury in the lamp falls within the range 0002:] to 0.1:] and an optimum ratio is 0.05:1. We have been able to operate such a lamp envelope at a greatly increased temperature compared with that of a lamp using mercury alone, to obtain a'corresponding increase in the watts radiated per unit volume over the ultra-violet waveband ranges and particularly in the region of the 254 nm and 366 nm lines.

This type of lamp can be used for photocopying purposes andprovides an enriched radiation in the ultraviolet waveband in a compact form, giving a substantial reduction in exposure time.

EXAMPLE I] A lamp rated at 2,000 watts has a fused silica envelope with an internal bore of 18 mm and an arc gap length of 190 mm. The electrodes are of the same construction as in Example I.

The lamp contains 18 mg of mercury, 0.5 mg ofmagnesium and 5 mg of mercuric iodide. The weight of mercury in the mercuric iodide is in proportion to the atomic weight of mercury divided by the atomic weight of mercuric iodide, so that the weight of mercury in the mercuric iodide is 5 X (200/454) mg 2.20 mg. The total weight of mercury in the lamp is thus 20.2 mg.

The internal lamp volume is 51 cc, and thus the total weight of mercury in the lamp is 0.396 mg per cc of lamp volume. Such a figure is lower than normally encountered in such a lamp and leads to lamp currents in excess of 15 amps. Nevertheless the voltage required across the lamp only needs to be about 125 volts, though the lamp will operate over a range of 95 volts to 160 volts. Thus the lamp canwithout difficulty be operated from a normal mains supply.

section. This contrasts with the usual medium pressure lamps of upwards of 1 atmosphere pressure in which the arc width is restricted with the result that on horizontal operation the arc is of so-called corded form and bows upwards, and consequently the top of the arc tube runs at a considerably higher temperature than the bottom. The mercury concentration used in the lamp of this example produces an arc of cross-section closely approaching that of the arc tube. This allows the lamp to be run at powers in excess of 250 watts per linear inch (which contrasts with 200 watts per inch for a conventional lamp) while still giving long lamp life well in excess of 500 hours, as the resultant lamp body temperature is much more uniform.

With a lamp design to produce visible radiation a constricted or cored arc produces light more efficiently, but in this instance an are which is operating at a lower pressure and which fills the arc tube instead of being corded will result in the discharge stream carrying the metal halide from the wall surface in to the arc discharge to give enhanced ultra-violet radiation as well as radiation from the low pressure mercury source being more efficient in the shorter wavebands.

The added magnesium in the quantity noted, namely 0.5 mg for about 20 mg of mercury considerably increases the amount of radiation in the 250.to 400 nm waveband by the contribution of line radiation from the magnesium added to that of the mercury spectrum. FIG. 2 shows the radiation spectrum of the lamp in the ultra-violet region. v

The lamp can then be used for the photopolymerisation of plastics, resins and ultra-violet inks.

The amount of mercury in the lamp can be varied up to about 0.5 mg/cc, corresponding at the operating temperature to approximately 1 atmosphere. Above this the arc tends to return to its more usual corded form and much of the increased efficiency is lost. Values below 0.396 mg/cc can also be used with increasing lamp currents resulting, and the only limitation in this direction is in the construction of the lamp and associated circuitry to handle the increased currents and temperatures. Normally therefore values below 0.27 mg/cc will be impracticable.

The amount of magnesium is related to the amount of mercury in the lamp. There must be sufficient magnesium to ensure the required broadening of the spectrum but there should not be an excess of magnesium in the tube. For this reason the weight ratio of magnesium to mercury should lie between 0.002:1 and 0.121, with values of 0.02 to 0.05:1 being preferred.

The lamp of this example thus provides improved ultra-violet emission at high power from a small source which can be run from a conventional mains supply. It has the advantage that it can operate either in the horizontal or vertical position as desired.

EXAMPLE I]! Such a mercury arc tube as described in Examples 1 and II containing a magnesium halide, of lower or higher wattage rating, can be designed for use as a source of visible radiation where the arc tube is contained within an outer envelope having a phosphor coating such as magnesium fiuorogermanate or yttrium vanadate to utilise the ultra-violet radiation generated and excite these phosphors to generate visible light.

The physical construction of the lamp may be similar ble light emanating from the phosphor providing a substantial improvement to the colour appearance and colour rendition properties over the normal phosphorcoated outer bulb containing a conventional mercury arc tube.

When the lamp is provided with -a phosphor-coated outer envelope, the lamp atmosphere can include other trace additives to improve the radiation in both the ultra-violet and visible wavebands, providing these additives typically in an amount of about 0.1 mg do not diminish the magnesium radiation or result in its absorption, which could occur if for example a sodium halide were to be used. Preferred additives are gallium and thallium halides, but the halides of the following metals could be used: aluminium, tin, iron, zinc, yttrium, chromium, bismuth and cadmium.

We claim:

1. An ultra-violet emitting mercury vapour discharge lamp, comprising:

an ultra-violet transmitting lamp envelop;

a discharge electrode system within said envelope;

and

an atmosphere withing said envelope, said atmosphere comprising mercury vapour and magnesium vapour in the form of a magnesium halide in such proportions as to provide enhanced ultra-violet emission, and an inert gas filling to initiate electron discharge, said magnesium being introduced into said lamp as magnesium metal together with another metal halide.

2. A lamp as claimed in claim 1, in which said other metal halide is a mercuric halide.

3. A lamp as claimed in claim 2, in which the weight of mercury in said envelope is below about 0.5 mg per cc of envelope volume.

4. A lamp as claimed in claim 1, in which said other metal halide is mercuric iodide.

5. A lamp as claimed in claim 1, in which the ratio by weight of magnesium to mercury is within the range 0.00221 and 0.121. Y

6. A lamp as claimed in claim 5, in which the weight of mercury in said envelope is below about 0.5 mg per cc of envelope volume.

7. A lamp as claimed in claim 1, in which the mercury pressure within said lamp is between about "A and about 1 atmospheres.

8. A lamp as claimed in claim 1, in which the weight of mercury in said envelope is less than about 0.5 mg per cc of envelope volume.

9. A lamp as claimed in claim 1,.said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising gallium and thallium.

10. A lamp as claimed in claim 1, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in whichsaid atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising aluminium, tin, iron, zinc, yttrium, chromium, bismuth and cadmium.

11. An ultra-violet emitting mercury vapour discharge lamp, comprising:

an ultra-violet transmitting lamp envelope; 7 a discharge electrode system within said envelope; 5 and an atmosphere within said envelope, said atmosphere comprising mercury vapour and magnesium vapour in the form of a magnesium halide in such proportions as to provide enhanced ultra-violet emission, arid an inert gas filling to initiate electron discharge, the ratio by weight of magnesium to mercury being within the range 0.002;] and 0.1 :1; whereby an enhanced broadband ultra-violet spectrum is obtained.

12. A lamp as claimed in claim 11, in which the mer cury pressure within said lamp is between about A and about 1 atmospheres.

l3. A lamp as claimed in claim 11, in which the about 0.5 mg per cc of envelope volume.

14. A lamp as claimed in claim 11, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of another metal halide.

15. An ultra-violet emitting mercury vapour discharge lamp, comprising:

an ultra-violet transmitting lamp envelope;

a discharge electrode system within said envelope;

and

an atmosphere within said envelope, said atmosphere comprising mercury vapour, magnesium vapour in the form of a magnesium halide, and an inert gas filling to initiate electron discharge, the ratio by weight of magnesium to mercury being within the range of about 0.02:1 to about 0.05:1;

whereby an enhanced broadband ultra-violet spectrum is obtained.

16. A lamp as claimed in claim 15, in which the mercury pressure within said lamp is between about 1 and about 1 atmospheres.

17. A lamp as claimed in claim 15, in which the weight of mercury within said envelope is less than about 0.5 mg per cc of envelope volume.

18. A lamp as claimed in claim 15, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of another metal halide.

19. An ultra-violet emitting mercury vapour discharge lamp, comprising! an ultra-violet transmitting lamp envelope;

weight of mercury within said envelope is less than a discharge electrode system within said envelope;

21. A lamp as claimed in claim 19,, in which the mercury pressure within said'lamp is between about and about 1 atmospheres.

22. A lamp as claimed in claim 19, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the groupcomprising gallium and thallium.

23. A lamp as claimed in claim 19, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising aluminium, tin, iron, zinc, yttrium, chromium, bismuth andcadmium.

24. A lamp as claimed in claim '19, in which the magnesium is introduced into said lamp as magnesium metal together with mercuric iodide.

25. A lamp as claimed in claim 24, in which the ratio.

7 8 fill comprises about 18 parts by weight of mercury,

about 0.5 partsby weight of mercuric iodide.

27. A mercury vapour discharge lamp comprising:

an ultra-violet transmitting lamp envelope;

a discharge electrode system within said envelope;

an atmosphere within said envelope, said atmosphere comprising mercury vapour, magnesium vapour in the form ofa magnesium halide, an inert gas filling to initiate electron discharge, and trace additions of another metal halide; and

a phosphor-coated outer envelope for emitting visible radiation.

28. A lamp as claimed in claim 27, wherein said mercury vapour is at a pressure of about 1 to l atmospheres.

29. A lamp as claimed in claim 27, wherein said another metal halide comprises a halide of at least one of the metals in the group comprising gallium and thallium.

30. A lamp as claimed in claim 27, wherein said another metal halide comprises a halide of at least one of the metals in the group comprising aluminum tin, iron,

zinc, yttrium, chromium, bismuth and cadmium. 

1. AN ULTRA-VIOLET EMITTING MERCURY VAPOUR DISCHARGE LAMP, COMPRISING: AN ULTRA-VIOLET TRANSMITTING LAMP ENVELOP; A DISCHARGE ELECTRODE SYSTEM WITHIN SAID ENVELOPE; AND AN ATMOSPHERE WITHING SAID ENVELOPE, SAID ATMOSPHERE COMPRISING MERCURY VAPOUR AND MAGNESIUM VAPOUR IN THE
 2. A lamp as claimed in claim 1, in which said other metal halide is a mercuric halide.
 3. A lamp as claimed in claim 2, in which the weight of mercury in said envelope is below about 0.5 mg per cc of envelope volume.
 4. A lamp as claimed in claim 1, in which said other metal halide is mercuric iodide.
 5. A lamp as claimed in claim 1, in which the ratio by weight of magnesium to mercury is within the range 0.002:1 and 0.1:1.
 6. A lamp as claimed in claim 5, in which the weight of mercury in said envelope is below about 0.5 mg per cc of envelope volume.
 7. A lamp as claimed in claim 1, in which the mercury pressure within said lamp is between about 1/4 and about 1 atmospheres.
 8. A lamp as claimed in claim 1, in which the weight of mercury in said envelope is less than about 0.5 mg per cc of envelope volume.
 9. A lamp as claimed in claim 1, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising gallium and thallium.
 10. A lamp as claimed in claim 1, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising aluminium, tin, iron, zinc, yttrium, chromium, bismuth and cadmium.
 11. An ultra-violet emitting mercury vapour discharge lamp, comprising: an ultra-violet transmitting lamp envelope; a discharge electrode system within said envelope; and an atmosphere within said envelope, said atmosphere comprising mercury vapour and magnesium vapour in the form of a magnesium halide in such proportions as to provide enhanced ultra-violet emission, and an inert gas filling to initiate electron discharge, the ratio by weight of magnesium to mercury being within the range 0.002:1 and 0.1:1; whereby an enhanced broadband ultra-violet spectrum is obtained.
 12. A lamp as claimed in claim 11, in which the mercury pressure within said lamp is between about 1/4 and about 1 atmospheres.
 13. A lamp as claimed in claim 11, in which the weight of mercury within said envelope is less than about 0.5 mg per cc of envelope volume.
 14. A lamp as claimed in claim 11, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of another metal halide.
 15. An ultra-violet emitting mercury vapour discharge lamp, comprising: an ultra-violet transmitting lamp envelope; a discharge electrode system within said envelope; and an atmosphere within said envelope, said atmosphere comprising mercury vapour, magnesium vapour in the form of a magnesium halide, and an inert gas filling to initiate electron discharge, the ratio by weight of magnesium to mercury being within the range of about 0.02:1 to about 0.05:1; whereby an enhanced broadband ultra-violet spectrum is obtained.
 16. A lamp as claimed in claim 15, in which the mercury pressure within said lamp is between about 1/4 and about 1 atmospheres.
 17. A lamp as claimed in claim 15, in which the weight of mercury within said envelope is less than about 0.5 mg per cc of envelope Volume.
 18. A lamp as claimed in claim 15, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of another metal halide.
 19. An ultra-violet emitting mercury vapour discharge lamp, comprising: an ultra-violet transmitting lamp envelope; a discharge electrode system within said envelope; and an atmosphere within said envelope, said atmosphere comprising mercury vapour, magnesium vapour in the form of a magnesium halide, and an inert gas filling to initiate electron discharge, the weight of mercury in said envelope being less than 0.5 mg per cc of envelope volume; whereby an enhanced broadband ultra-violet spectrum is obtained.
 20. A lamp as claimed in claim 19, in which the ratio by weight of magnesium to mercury is within the range 0.002:1 and 0.1:1.
 21. A lamp as claimed in claim 19, in which the mercury pressure within said lamp is between about 1/4 and about 1 atmospheres.
 22. A lamp as claimed in claim 19, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising gallium and thallium.
 23. A lamp as claimed in claim 19, said lamp further having a phosphor-coated outer envelope for emitting visible radiation, and in which said atmosphere in said lamp further comprises trace additions of a halide of at least one of the metals in the group comprising aluminium, tin, iron, zinc, yttrium, chromium, bismuth and cadmium.
 24. A lamp as claimed in claim 19, in which the magnesium is introduced into said lamp as magnesium metal together with mercuric iodide.
 25. A lamp as claimed in claim 24, in which the ratio by weight of magnesium to mercury is within the range 0.02:1 and 0.05:1, and the mercury pressure within said lamp is between about 1/4 and about 1 atmospheres.
 26. A lamp as claimed in claim 25, in which the lamp fill comprises about 18 parts by weight of mercury, about 0.5 parts by weight of mercuric iodide.
 27. A mercury vapour discharge lamp comprising: an ultra-violet transmitting lamp envelope; a discharge electrode system within said envelope; an atmosphere within said envelope, said atmosphere comprising mercury vapour, magnesium vapour in the form of a magnesium halide, an inert gas filling to initiate electron discharge, and trace additions of another metal halide; and a phosphor-coated outer envelope for emitting visible radiation.
 28. A lamp as claimed in claim 27, wherein said mercury vapour is at a pressure of about 1/4 to 1 atmospheres.
 29. A lamp as claimed in claim 27, wherein said another metal halide comprises a halide of at least one of the metals in the group comprising gallium and thallium.
 30. A lamp as claimed in claim 27, wherein said another metal halide comprises a halide of at least one of the metals in the group comprising aluminum tin, iron, zinc, yttrium, chromium, bismuth and cadmium. 